Early life conditions influence fledging success and subsequent local recruitment rates in a declining migratory songbird, the Whinchat Saxicola rubetra

Abstract Life history traits and environmental conditions influence reproductive success in animals, and consequences of these can influence subsequent survival and recruitment into breeding populations. Understanding influences on demographic rates is required to determine the causes of decline. Migratory species experience spatially and temporally variable conditions across their annual cycle, making identifying where the factors influencing demographic rates operate challenging. Here, we use the Whinchat Saxicola rubetra as a model declining long‐distance migrant bird. We analyse 10 years of data from 247 nesting attempts and 2519 post‐fledging observations of 1193 uniquely marked nestlings to examine the influence of life history traits, habitat characteristics and weather on survival of young from the nestling stage to local recruitment into the natal population. We detected potential silver spoon effects where conditions during the breeding stage influence subsequent apparent local recruitment rates, with higher recruitment for fledglings from larger broods, and recruitment rate negatively related to rainfall that chicks experienced in‐nest. Additionally, extreme temperatures experienced pre‐ and post‐fledging increased fledging success and recruitment rate. However, we could not determine whether this was driven by temperature influencing mortality during the post‐fledging period or later in the annual cycle. Brood size declined with hatching date. In‐nest survival increased with brood size and was highest at local temperature extremes. Furthermore, nest survival was highest at nests surrounded with 40%–60% vegetation cover of Bracken Pteridium aquilinum within 50 m of the nest. Our results show that breeding phenology and environmental factors may influence fledging success and recruitment in songbird populations, with conditions experienced during the nestling stage influencing local recruitment rates in Whinchats (i.e. silver spoon effect). Recruitment rates are key drivers of songbird population dynamics. Our results help identify some of the likely breeding season mechanisms that could be important population drivers.

characteristics and weather on survival of young from the nestling stage to local recruitment into the natal population. We detected potential silver spoon effects where conditions during the breeding stage influence subsequent apparent local recruitment rates, with higher recruitment for fledglings from larger broods, and recruitment rate negatively related to rainfall that chicks experienced in-nest. Additionally, extreme temperatures experienced pre-and post-fledging increased fledging success and recruitment rate. However, we could not determine whether this was driven by temperature influencing mortality during the post-fledging period or later in the annual cycle.
Brood size declined with hatching date. In-nest survival increased with brood size and was highest at local temperature extremes. Furthermore, nest survival was highest at nests surrounded with 40%-60% vegetation cover of Bracken Pteridium aquilinum within 50 m of the nest. Our results show that breeding phenology and environmental factors may influence fledging success and recruitment in songbird populations, with conditions experienced during the nestling stage influencing local recruitment rates in Whinchats (i.e. silver spoon effect). Recruitment rates are key drivers of songbird population dynamics. Our results help identify some of the likely breeding season mechanisms that could be important population drivers.

| INTRODUC TI ON
Across Europe many migratory bird species have undergone substantial population declines over recent decades, with long-distance Afro-Palearctic migrants experiencing particularly severe declines (Vickery et al., 2014). Although the causes remain largely undiagnosed, declines could be driven by changes in productivity or impacts on survival at one or more locations across their annual cycle, that is, at breeding sites, non-breeding sites or during migration (Howard et al., 2020;Vickery et al., 2014). Furthermore, conditions individuals experience during one stage can affect productivity and survival rates in subsequent stages, that is, carry-over effects (Harrison et al., 2011). Carry-over effects are typically considered non-lethal effects; however, they can apply to scenarios where conditions experienced during one stage influence survival rates in subsequent seasons (Harrison et al., 2011). By this definition, if conditions experienced during early life influenced survival in subsequent seasons of the annual cycle this effect would be considered both a silver spoon (Cockburn, 1991;Grafen, 1988) and carry-over effect. Carry-over effects have been detected in many migratory birds, particularly effects linking non-breeding (Rushing et al., 2016) and migratory conditions (Finch et al., 2014) to reproductive stages.
After hatching, many bird species experience high pre-fledging mortality, and although less studied, the period between leaving the nest (fledging) and independence from parental care is also often a period with high mortality (Cox et al., 2014;Naef-Daenzer & Grüebler, 2016). For declining migratory species, disentangling post-fledging mortality from first year mortality is difficult but can help identify whether pressures occur in breeding or non-breeding areas, which is useful knowledge for guiding conservation effort.
Migratory birds face potential impacts on survival at multiple stages and locations across their annual cycle, with each stage and location a potential survival bottleneck (Faaborg et al., 2010). Therefore, migrants are more likely to experience environmental change which could impact survival ('Multiple jeopardy' ;Newton, 2004).
In this study, we combine 10 years of detailed breeding data from 247 nests and resighting data from 1193 individually marked nestling Whinchats Saxicola rubetra to examine the influence of environmental drivers on stage specific survival and local recruitment. Like many other Afro-Palearctic migrants, Whinchat populations have decreased substantially over recent decades, 89% since 1980 across Europe (PECBMS, 2020) and 57% since 1995 in the UK (Woodward et al., 2020). Previous studies indicate that breeding productivity and dispersal dynamics are key drivers of population change (Fay et al., 2021), but the mechanisms remain poorly understood. We investigate the effects of natal habitat and weather on in-nest survival (i.e. fledging success) and local recruitment into the breeding population (i.e. juvenile survival). We also test whether pre-and postfledging conditions influence subsequent rates of local offspring recruitment into the population via a silver spoon effect.

| Study area
Our study was conducted between 2013 and 2022 at RSPB Geltsdale nature reserve in the North Pennines in Cumbria, UK (54.9° N-2.6° S), which is jointly owned by the Royal Society for the Protection of Birds and the Weir Trust. The survey area is an ~11 km 2 sub-section of the reserve comprising blanket bog, heathland and acid grassland, with an altitude of 220-440 m.

| Study species and field methods
Whinchats are short-lived (<8 years) Afro-Palearctic migrants, breeding in grassland habitats throughout Europe and Western Asia and migrating annually to sub-Saharan Africa for the northern winter.
Whinchats are ground nesting and usually lay a single clutch of 4-7 eggs. The incubation period is 12-14 days with young provisioned by both parents for ~13 days before fledging. Young are capable of flight 3-5 days after fledging, with a further 9-15 days spent close to their natal nest while they are still dependent on their parents for food (Collar, 2005;Tome & Denac, 2012). Post-independence, fledglings typically remain in their natal area for 1-2 months, when they undergo a partial moult prior to southerly migration (Collar, 2005).
We began searching for nests when Whinchats arrived at Geltsdale in May, with searches performed almost daily until nesting had ceased in July. Nests were located by observing adult behaviour (male singing, nest building, guarding and incubating). Males are typically more conspicuous than females during the breeding season, so the male of a pair was usually identified first, but once a nest was located females were also identified. We visited each nest every 3-7 days, recording clutch size, brood size and to confirm number fledged (see Table 1 for definitions). Whinchat may have replacement

K E Y W O R D S
Afro-Palearctic, brood size, migration, post-fledging survival, rainfall, silver spoon effect

T A X O N O M Y C L A S S I F I C A T I O N
Applied ecology, Conservation ecology, Landscape planning, Life history ecology, Population ecology nests if the first fails, which typically have smaller clutches (5.4 vs. 3.4: Müller et al., 2005, 6.8 vs. 5.8: Shitikov et al., 2015 and lower fledging success (Grüebler et al., 2015). In most cases, we could not reliably determine whether a nest was a first or replacement attempt, or a second nest after the success of the first. We estimated first egg laying date through back-calculations from either observation of incomplete clutches assuming one egg is laid daily, or for nests found post-hatching, by back-calculating based on chick development stage assuming 14 days of incubation and a clutch size equal to brood size plus the number of unhatched eggs. All chicks were ringed with a unique combination of three colour rings and a numbered metal ring 6-8 days post-hatching. Fledging success was usually determined from resighting of fledglings. Fledging date was estimated from chick development stage observed during nest visits. For our analyses, we used nest visit data to determine how many chicks successfully hatched and whether a nest successfully fledged at least one chick. Nest visits and bird handling were undertaken by field workers with ringing permits granted by the British Trust for Ornithology, and to minimise nest disturbance no active nest was intrusively monitored on more than four occasions.

(b) Response variables Description
Nest success Binary factor denoting whether a nest successfully fledged at least one chick. Modelled with a binomial distribution.
Local recruitment Binary factor designating whether at least one fledgling was resighted on their natal site in subsequent years. Modelled with a binomial distribution.
Proportion recruited Number of resighted fledglings as a proportion of the total fledged per nest. Modelled with a binomial distribution (mean: 0.25; range 0-1).

(c) Random effect Description
Year Unique factor designating the year that a nest was sampled. Used throughout to account for between year variation in offspring survival.
Note: Details of fixed effects (a), response variables (b), and random effects (c) used throughout analysis.
in the season fledglings and adults congregated to moult in certain areas, and these hotspots were surveyed more frequently in August and September.
Despite rigorous and frequent searches of the field site, some nesting attempts would inevitably have been missed. Because failed nests are active for a shorter period than successful nests, they are more likely to be missed, so a direct estimate of nest success rates from failed versus fledged nests may overestimate fledging success.
To account for this, we performed a nest survival analysis (Dinsmore et al., 2002;Mayfield, 1975), which estimated the probability of nest survival from the total number of days that each nest survived (i.e. exposure days). Using this approach, we estimated a nest survival rate of 73.3%, which is slightly lower than the direct estimate from our data (80.6%). However, this is unlikely to affect our assessment of the systematic factors influencing nest survival unless these factors also influenced the likelihood of observers finding a nest. Given that nests were usually found by locating calling adults, we find it unlikely that any of the key factors we investigate (e.g. vegetation, weather) should affect the likelihood that a nest failed prior to being located. For further details on survival analysis see supplementary material.

| Vegetation and habitat sampling
The vegetation substrate on which a nest was built and the vegeta- Holcus lanatus). These measures were then used to calculate the relative percentage cover by each vegetation type within each area.
The number of trees and presence or absence of key features (e.g. fence post, wall) was also recorded within each radius; for the 5 m radius the number of trees was counted exactly, whereas for 50 m and 200 m radii the number of trees were estimated and binned.
For further details on habitat sampling, see Table 1 and Table S1 (Appendix S1). Additionally, the elevation at which each nest was located was recorded.

| Analysis
All analyses were performed using R 4.0.2 (R Core Team, 2020).
Because vegetation data was sampled during a subset of years  Tables S2-S4).
Second, two-term GLMMs were used to assess which continuous variables should be included as quadratic variables. Any quadratic variables with p ≤ .10 were considered for inclusion in subsequent models as both quadratic and linear variables. Finally, these variables were checked for correlations, and where two variables were highly correlated with each other (Spearman rank correlation coefficient, |r s | ≥ .70, Dormann et al., 2013) the more significant predictor (i.e. the smaller p-value) was included in the full model (Tables S5-S6). This process was repeated for each response variable. For the analysis of data from all years, all weather and life history variables were included in the full model as both linear and quadratic terms (Tables S7-S9) unless highly correlated (|r s | ≥ .70) with a more significant term (Tables S10-S11).
Once the number of explanatory variables was reduced, we  Table 2, full model outputs are available in Tables S12-S17.  . Model terms were selected using a two-stage modelling approach, as follows: each term was modelled in a single term model and terms with p-values < .10 were considered for inclusion in the full model unless highly correlated (|r s | > .70) with another term.

| Nest success
Stepwise elimination of terms from this full model was then performed until only significant terms remained, leaving a minimum adequate model (MAM, terms in bold). All models were run including the random effect 'Year'. Local recruitment refers to whether a nest successfully had ≥1 fledgling resighted in subsequent years. Sample sizes for resighting analysis are smaller than analysis of fledging success because two nests were omitted due to missing fledge date, with weather during the post-fledging period not possible to use. All models were fitted with a binomial distribution.
removal of either the Bracken or Tree scrub term resulted in the other being significant, though neither were significant when modelled together. The all-years MAM (N = 247) also found that fledging success increased with brood size (GLMM: p < .001, Table 3b, Figure 1c), and found a significant positive effect of altitude on nest success (GLMM: p = .024, Table 3b, Figure 1d).  Predicted relationships (± 95% CI) are fitted from generalised linear mixed effects models, see Table 3.

TA B L E 4
Generalised linear mixed effects models outputs for variables influencing whether a nest had at least one fledgling recruited.  Figure 2b). The all-years MAM also revealed that, as expected, the probability of a successful nest producing at least one local recruit also increased with brood size (GLMM:

| DISCUSS ION
In summary, we found that larger Whinchat broods: (i) occurred most in the early-mid stage of the breeding season, (ii) were more likely to fledge and (iii) showed higher rates of apparent survival to recruitment. Importantly, fledglings that experienced higher rainfall prior to fledging had lower apparent survival to recruitment, suggesting a silver spoon effect where conditions experienced in-nest influenced subsequent local recruitment probability. Nests that experienced temperature extremes prior to fledging were more likely to fledge, and fledglings that experienced temperature extremes in the post-fledging period had higher apparent survival to recruitment.

F I G U R E 2
The effect of key variables on the likelihood of a nest producing one recruit. The probability of at least one fledgling from a successful nest being resighted in subsequent years against (a) brood size, from all years (N = 197 nests); and (b) percentage Heather cover within 200 m of the nest, from years when vegetation data were collected (N = 118 nests). Predicted relationships (± 95% CI) are fitted from generalised linear mixed effects models, see Table 4.

TA B L E 5
Generalised linear mixed effects models outputs for variables influencing the proportion of fledglings recruited per nest.
Additionally, nests with mid-range Bracken cover (50%) were most likely to fledge, with other microhabitat features influencing apparent survival to recruitment to a lesser extent.

| Life history traits
In addition to producing more fledglings, larger broods were also more likely to successfully fledge. For migratory birds, favourable conditions in non-breeding locations are often associated with earlier arrival to breeding sites, earlier egg laying and higher productivity (e.g. Ockendon et al., 2013), including for Whinchat (Grüebler et al., 2015;Müller et al., 2005;Shitikov et al., 2015). A relationship between the timing of breeding and clutch size, and thus brood size, is well established in Whinchat (Fuller & Glue, 1977;Grudinskaya et al., 2022), including in our study, where brood size peaked for nests that hatched in the early-middle period of the breeding season, then declined over time (Table S18, Figure S3). This effect may be partly due to parental fitness or experience because higher quality and/or more experienced individuals may arrive earlier to the breeding site, produce larger broods, and exhibit higher reproductive success. Additionally, Whinchat may produce replacement broods in the event of nest failure, which relative to their first, typically have smaller clutches (5.4 vs. 3.4: Müller et al., 2005, 6.8 vs. 5.8: Shitikov et al., 2015 and lower fledging success (Grüebler et al., 2015). Also, although not well documented for Whinchat, it is occasionally possible for early breeding pairs to have a second brood after the success of their first. We con- Predicted relationships (± 95% CI) are fitted from generalised linear mixed effects models, see Table 5.
rates of renesting in our population. None the less, renesting likely contributed to the peak in brood size in the early-mid stage of the season (two apparent peaks in nesting activity; Figure S3), and to the positive relationship between brood size and nest success. In a previous study, lower survival of replacement Whinchat broods was a consequence of timing differences within the season rather than intrinsic differences between first and replacement nests (Grüebler et al., 2015). Therefore, Whinchat broods should experience the same survival challenges regardless of parental quality, experience or whether they were replacement nests, so these results still require a mechanism by which larger or later broods have lower reproductive success. Daily in-nest survival rates are often higher in earlier broods (Low & Pärt, 2009), possibly due to seasonal increases in predation threat (Hatchwell, 1991;Verhulst & Nilsson, 2007). Earlier fledging can lower rates of post-fledging predation (Naef-Daenzer & Grüebler, 2016;Verhulst & Nilsson, 2007), and pre-fledging conditions can influence post-fledging survival; with fledglings from earlier broods seemingly less susceptible to mortality in subsequent stages (Naef-Daenzer & Grüebler, 2016). Furthermore, higher success for larger broods is unlikely to be due to buffering against complete failure due to cumulative events of mortality, as Whinchat experience very low rates of partial brood failure (7.3% in our study; 3% in Border, Henderson, Ash, et al., 2017), meaning that the number of fledglings was highly correlated with brood size (r s = .827).
We used resighting rates of fledglings returning as adults to estimate apparent survival to recruitment, so we could not distinguish mortality from dispersal to outside our study area. Therefore, we cannot exclude the possibility that natal conditions, for example, brood size, fledging date, or whether they were from a replacement brood, differentially affect dispersal movements. However, we recorded a high rate of nestlings recruiting into our study population (20%), which coupled with low natal dispersal distance (Shitikov et al., 2012) and high adult high site fidelity in Whinchat (Blackburn & Cresswell, 2016), gives confidence to our findings on recruitment rate. For example, we found that fledglings from larger broods had a higher apparent recruitment rate. Clutch size, and hence brood size, is predominantly related to pre-laying environmental conditions experienced in the breeding area (Haywood & Perrins, 1992), breeding phenology (Müller et al., 2005;Shitikov et al., 2015) and parental fitness (Smith & Moore, 2003). In Whinchat, a carry-over effect could influence breeding phenology, for example, a higher mass upon departure from African non-breeding sites can correspond with earlier departure (Risely et al., 2015). We suggest that the observed positive effect of brood size on reproductive success is driven largely by higher reproductive success for earlier timed nests, which could be mediated by seasonal variation in predation risk and/or variation in the length of the post-fledging period prior to migration.
However, we note that parental quality or experience, differences between first and replacement nests, and carry-over effects from non-breeding site conditions can also influence reproductive success, both directly and indirectly via the timing of breeding. It is also possible that positive selection for earlier breeding may in part also arise from higher rates of recruitment by young from earlier nests as an endogenous mechanism of heritable timing (Akresh et al., 2021;Bazzi et al., 2016;Saino et al., 2015;Sosnovcová et al., 2018).

| Weather
Temperature extremes (i.e. <13°C or >15°C) experienced during the in-nest period resulted in a higher likelihood of fledging, and a higher apparent recruitment rate. This is surprising given that daily nest survival decreases linearly with temperature for many passerines (e.g. Low & Pärt, 2009;Tome et al., 2020). Rates of nest predation can also increase  or decrease (Skagen & Yackel Adams, 2012)  and raptors (e.g. common buzzard, Buteo buteo) (Frankiewicz, 2008;Tome & Denac, 2012). Exactly why mid-range temperatures predict high in-nest and post-fledging mortality is not clear, but we speculate that this may be mediated by variation in predator activity or efficiency of predator foraging at different temperature extremes.
For example, at cold temperatures chicks may require more provisioning to maintain homeostasis and thus may beg louder or more frequently (Kilner & Johnstone, 1997) potentially attracting auditory predators. Likewise, during the post-fledging period recently independent fledglings may have to take more risks when foraging at low temperatures (Hilton et al., 1999). Whereas, at high temperatures, favourable temperature and/or light conditions may increase hunting activity of predators .
Fledglings that experienced higher rainfall prior to fledging had lower apparent survival to recruitment. Weather conditions, especially rainfall, are known to increase pre-and post-fledging mortality of passerines, but in our study rainfall prior to fledging influenced apparent recruitment, suggesting that conditions Whinchat fledglings experienced in-nest influenced survival during subsequent stages. However, as we could not determine whether this effect was mediated by a change in mortality during the post-fledging period (i.e. intra-seasonal) or during subsequent stages of the annual cycle (i.e. inter-seasonal) we are unable to conclude whether this was a true carry-over effect or just a silver spoon effect. Similar effects have been found in other passerines, with better condition fledglings experiencing higher post-fledging survival (Naef-Daenzer & Grüebler, 2016;Vitz & Rodewald, 2011).
High rainfall prior to fledging may increase thermoregulatory costs to nestlings (Visser et al., 1998) or reduce prey availability for parents provisioning their offspring and hence slow provisioning rate, thus diminishing offspring condition (Öberg et al., 2015;Radford et al., 2010).

| Vegetation
The only vegetation term to significantly impact nest success was Bracken cover within 50 m of the nest, which had a quadratic relationship with likelihood of fledging, with success highest at around 50% cover. It is perhaps unsurprising that Bracken was associated with nest success, given that Whinchat breeding in the UK frequently associate with high proportions of Bracken cover (Allen, 1995;Pearce-Higgins & Grant, 2006;Stanbury et al., 2022). However, the quadratic nature of the relationship is interesting, with fledging success declining above 50% Bracken cover. Whinchats preferentially nest in areas with high structural vegetation diversity (Border, Henderson, Redhead, et al., 2017;Douglas et al., 2017;Fischer et al., 2013), which are associated with greater quality, diversity and abundance of invertebrate prey (Britschgi et al., 2006;Evans et al., 2015) but avoid foraging in areas with high Bracken cover (Murray et al., 2016). Therefore, a trade-off may exist between nest concealment offered by high Bracken cover and the low foraging efficiency of Bracken monocultures.
Tree scrub cover within 200 m of the nest also showed a marginally non-significant negative trend with likelihood of fledging, and the removal of the Bracken 50 m term from the model resulted in Tree scrub cover being significant. We attribute this to the negative correlation between Bracken and Tree scrub cover (r s = −.41, Table S5). Similarly, analysis of all years, which did not include vegetation measures, found that the likelihood of fledging increased with altitude, which we suggest is driven by a weak positive correlation between altitude and Bracken cover (50 m; r s = .38, Table S5).
A review by Cox et al. (2014)

| CON CLUS ION
Our study provides evidence of potential silver spoon effects from natal brood size and rainfall during the in-nest period to subsequent survival to recruitment into the local breeding population in a migratory passerine. However, it is not known whether this effect was mediated by a change in survival during the post-fledging period (i.e. intra-seasonal) or during subsequent stages (i.e. inter-season), thus we cannot determine whether this is an example of a true carry-over effect. Similar effects detected in other migratory birds mostly occur within an annual cycle, linking non-breeding or migration conditions to reproduction within the same annual cycle. However, we appear to detect some sustained influence of early-life effects on mortality rate during subsequent stages, which could be especially crucial for maintaining population persistence (i.e. a silver spoon effect). This highlights the importance of identifying the drivers of juvenile survival and local recruitment for declining migratory birds, and the role of early-life conditions, to better understand the mechanisms.

ACK N O WLE D G E M ENTS
We thank Adam Moan, Ellie Ames, Joan Castello, Judit Mateos and Tina Wiffen for their valuable contributions to field work.

This work was supported by the Natural Environment Research
Council (NE/S00713X/1).

CO N FLI C T O F I NTE R E S T S TATE M E NT
The authors declare no conflict of interest.

O PE N R E S E A RCH BA D G E S
This article has earned Open Data and Open Materials badges. Data and materials are available at https://doi.org/doi:10.5061/dryad. msbcc 2g3p.

DATA AVA I L A B I L I T Y S TAT E M E N T
All data and code used in this analysis are available from https://doi.